1. Technical Field
The present disclosure relates to demodulators for digital transmissions, and more specifically to the blind search of transmission channels present in a modulated signal.
2. Description of the Related Art
FIG. 1 schematically shows several television channels carried on a modulated signal of a satellite transmission. The useful bandwidth for the channels is illustrated between 950 and 2150 MHz. A first channel has a central frequency Fc1 and a bandwidth Fs1. The second channel has a central frequency Fc2 and a bandwidth Fs2. Other channels have respective central frequencies and respective bandwidths. The central frequency of a channel is also the modulation frequency used for the signal transmitted in the channel. The bandwidth at half-power corresponds to the data rate of the channel, and it is usually equal to the symbol frequency, a “symbol” being the portion of a signal corresponding to one or several bits, according to the type of modulation used. The symbol frequency can conventionally reach 50 MBaud.
As shown for the first three channels, a channel is supposed to have a standardized envelope, trapezoidal and symmetric with respect to the central frequency. Moreover, the rising and falling edges transition smoothly into the horizontal portions. The signal amplitude varies from one channel to the other.
Many satellite channels, used by well-established content providers have a known and invariable central frequency and symbol frequency. Demodulation equipment can thus be preset to receive these channels without the intervention of the user.
Nevertheless, users may wish to receive all the programs available on the satellites. It is then necessary to find channels whose parameters are unknown or variable, used by providers who are inconsistent or do not publish the parameters. The demodulator should in this case be capable of a “blind” search for the available channels, i.e., finding the channels without knowing their parameters.
For a demodulator to lock on a channel, it identifies the central frequency and the symbol frequency of the channel. A blind seek is thus a two-dimensional search.
A brute-force method, using successive trials, includes adjusting the symbol frequency by increments and scanning the whole range of central frequencies for each increment of the symbol frequency. For each trial, the demodulator stabilizes and several thousand symbols should be transmitted in order to ascertain that the current setting corresponds to a channel. The frequency increments are limited to the capture range of the demodulator, both for the central frequency and the symbol frequency. Considering conventional transmission constraints, the capture range is in the order of 1% of the adjusted frequency. The number of trials attempted to scan all the possibilities with such a small step takes a long time.
United States Patent Application No. 2006/0044477 discloses a method for accelerating the search, based on the standardized shape of the envelopes of the channels. It consists in sliding a window across the range of central frequencies and to find, by spectrum analysis in the window, the centers of the edge portions of the envelopes. The central frequencies are then estimated half way between the thus identified edges.
However, due to the less than optimal transmission and reception conditions increasingly encountered, the channels, like the last one shown in FIG. 1, may be distorted such that their envelopes no longer conform to the standards. Such channels cannot be found with the solution disclosed in the above-mentioned patent application.